Editor’s note: This article was written by Alejandro R. Castillo, extension dairy specialist with the University of California Cooperative Extension and first appeared in the California Dairy Newsletter.

An on-farm survey was carried out on 40 commercial dairy farms in Merced County, California to study the effects of feed and feeding management variables on feed conversion (FC) and nitrogen utilization efficiency (NUE). The dairies were selected based on total salt (TS) content in drinking water and milk yield (MY) per cow. A lineal correlation analysis was used to study the association of different variables related to production efficiency, feed quality, and feed management practices. These variables were: FC, NUE, MY per cow, daily dry mater intake (DMI) per cow, total mixed ration (TMR) nutrient and mineral content, dietary cation-anion difference (DCAD), number of TMRs per farm, number of lactating dairy cows per farm, crude protein balance (CPB) estimated by the difference among total crude protein (CP) supply & CP required, and water TS content.

Results of the correlation analysis are presented in Table 1. Only significant correlations (r>0.30 and P<0.05) are reported. The table shows those variables with positive and negative effects on FC and NUE. The positive variables were ranked according to the magnitude of “r” and “P” values.

Summarizing, the three most important variables positively associated with FC and NUE were: milk yield per cow, number of TMR for lactating cows, and dietary energy content as non-fiber carbohydrates (NFC). Excess CP in the diet negatively affected both, FC and NUE. The dietary %CP and %NDF did have individual negative impacts on NUE and FC, respectively.

Another interesting finding from the study was that water TS content affected the DCAD value. The DCAD estimation should include minerals from the water when TS in drinking water is more than 500 mg per liter. It was estimated in this study that drinking water with a mean of 800±310 mg TS per liter provided on average 7 percent of calcium, 9 percent of magnesium, 25 percent of chloride, 25 percent of sodium, and 8 percent of sulfur dairy cow requirements (producing 70 lb of milk per day).